This invention relates to a compressor rotor wheel, and more particularly to a centrifugal compressor rotor wheel, for turbomachines the blade portions of which are made of a heat-resistant metallic material and the rotor disk portions of which are made of a high-strength metallic material, where a coaxially extending steel connection is provided between the blade region and the disk region.
Small gas turbine engines of both the turbojet and the turboshaft configurations are normally fitted with centrifugal or combined axial-radial flow compressors. While such engines may occasionally come recommended for their rather attractive specific fuel consumptions and power weight ratios their acceleration capability at abrupt load changes often leaves room for improvement. This is attributed to the fact that the polar moment of inertia of the rotor, by its very conception, is higher than with gas turbine engines having a straight axial-flow compressor, where the centrifugal cmpressor normally carries the largest single share.
Various recommendations made in an attempt to reduce the polar moment of inertia of centrifugal compressor rotor wheels seek their solutions in breaking the monolithic rotor wheel down into several assemblies, where e.g. the bladed shell is self-supported and is connected to the hub region by no means other than flexible and/or interlocking elements. There are obvious limitations on the use of this construction at elevated speeds and temperatures.
Other solutions have been proposed in which the monolithic rotor wheel is broken down into blades, the shell carrying the blades, and disks absorbing radial forces, where the various constituent parts are optimized for their specific functions and are then brazed together. Owing to the brazing properties of the materials lending itself to the purpose, all constituent parts are necessarily made of high-strength, i.e. similar materials. This means, however, that the outer and more moderately stressed zones, which nevertheless cause the major portion of the moment of inertia of a centrifugal compressor rotor wheel, must equally be made of steel although materials of less density, such as titanium, would fully do the job at these locations.
In a broad aspect the present invention provides a rotor wheel of said description which, using dissimilar materials for the blade region and for the disk region that cannot be brazed and/or conventionally welded together, will be low in mass and inertia.
It is a particular object of the present invention to provide a compressor rotor wheel, especially a centrifugal compressor rotor wheel which is low in mass and inertia by breaking the monolithic wheel into two or more regions, where the blades and the shell carrying them are manufactured as an integral part from a suitable heat-resistant alloy of low density, such as a titanium alloy, and where the load-bearing disk-like parts are made of a high-strength material, such as a martensitic steel alloy. In this arrangement the parts are joined together via one each, conceivably segmented spacer ring of steel explosion welded to the bladed shell and then brazed to the load-bearing disk-like parts or conceivably connected to these parts by fusion or friction welding.
The use of the explosion welding process to join steel and titanium alloys together is being practiced in the construction of chemical apparatus, where it is used to produce perfect surface area connections of relatively thin titanium panels to steel components. Explosion welding has also been cited in German patent specification 25 10 286.
The extremely brittle zones known from various experiments exploring the intimate connection of steel to titanium parts by brazing and diffusion, friction or electron beam welding to occur in the joint area are avoided because the welding process is extremely fast.
In a further aspect of the present invention the explosion welded joints are made before the bladed steel is machined, said shell being optionally machined from the solid, cast or manufactured powder metallurgically. Should explosion welding cause inaccuracies, these can be eliminated in the course of mechanical machining aimed at preparing the abutting areas between the spacer ring and the load-bearing disk-like parts. These corrections may conceivably be made also when the rotor wheel is finish machined, when especially the rear wall of the wheel is worked down mechanically or electrochemically in the outer diameter area to a minimum wall thickness imposed by the risk of deformation and/or by manufacturing requirements.
In a further aspect of the present invention the load-bearing disk-like parts are made of a high-strength titanium alloy, where the said parts are joined to the bladed shell via two, conceivably segmented spacer rings of a steel alloy, the rings being first joined by explosion welding to the respective adjacent component and then being brazed or conceivably welded together.
The purpose of the steel spacer rings between the two major assemblies made of dissimilar titanium alloys is to avoid brittle areas that would be inevitable when the parts are welded together. When they are brazed together, the difficulties resulting from the propensity of titanium for oxidation are eliminated, and the problems posed by the unlike heat treatments used in the aging of the finished assembly composed of dissimilar titanium alloys are alleviated by suitable selection of the braze alloy and the brazing temperature.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.